1. Field of the Invention
This invention relates to methods of making new and improved multi-layer coatings for gas turbine engine parts that are exposed to elevated temperatures, such as blades and vanes in the high pressure compressor and turbine of multi-stage aircraft engines. The invention comprises application of a diamond film over thermal barrier coatings such as yttria-stabilized zirconia, improving the erosion resistance of the thermal barrier coating.
2. Background of the Invention
This invention relates to improving the erosion resistance of thermal barrier coatings used on certain gas turbine engine parts such as combustion chambers, transition ducts, afterburner liners, blades and vanes. Such parts are exposed to very high temperatures during engine operation and to thermal cycling when the engine is started up or shut down. Yet, still higher gas temperatures lead to increased thermal efficiency of the turbine engines themselves. There is a need, therefore, for further improvements in thermal barrier coatings for critical parts of such engines.
It is known to protect critical gas turbine engine parts with thermal barrier coatings made of a variety of low thermal conductivity refractories. U.S. Pat. No. Re. 33,876 (Goward, et al.), for example, discloses plasma spray coatings of refractories comprising zirconia, preferably stabilized in its cubic form by addition of Yttria, La.sub.2 O.sub.3, calcium oxide or magnesium oxide. Such coatings typically are desired to be applied over nickel- or cobalt-based superalloy such as Hastelloy X, TD-nickel or Haynes 188. Because of mismatch between the coefficients of thermal expansion of the refractory ceramic coating and the alloy, however, an intermediate bond coating of alloys of the base metal with Cr, Al and Y (termed MCrAlY bond coatings) are used to help reduce spa ling of the refractory. Typically a very thin surface layer of such MCrAlY bond coatings is in oxidized form. In addition, the concentration of stabilized zirconia in the refractory coating may be graded continuously, from zero adjacent the MCrAlY bond coat up to 100% at the external surface of the refractory. The foregoing technique, however, produces discrete metal alloy particles within the refractory oxide coating. In use, oxidation of the metal particles can increase their size, causing unacceptable stress in the refractory coating.
Another proposed solution to the problem of thermal expansion coefficient mismatch between the superalloy and the refractory coating is to segment the refractory coating in some fashion, for example by means of bonding tiles or other discrete shapes of refractory to the metal. In this approach, which is generally applied to large articles, the segments are not bonded to each other, and the gaps between the tiles permit accommodation of the thermal expansion of the metal. Such an approach (the application of individual segments) would not be practical in the case of gas turbine engine components in which extreme operating conditions will be encountered and which a multiplicity of small complex parts must be coated at a reasonable cost. Additionally, in the use of such a segmented ceramic approach, there still remains the problem of obtaining a good metal-ceramic bond.
Still another approach to the application of thermal barrier coatings to gas turbine engine parts has been to use physical vapor deposition, for example electron beam physical vapor deposition (EBPVD) (a technique described in U.S. Pat. No. 5,087,477) to create a ceramic coating is comprised of many individual columnar segments which are firmly bonded to the article to be protected, but not to each other. By providing gaps between the columnar segments, such EBPVD refractory coatings are said to allow the metallic substrate to expand during engine operation without causing damaging stresses in the ceramic. Such columnar refractory coatings are disclosed in U.S. Pat. No. 4,321,311, which also suggests applying them over an MCrAlY bond coat having a thin oxidized surface. Preferable refractory ceramics for EBPVD application with columnar morphology include zirconia (preferably stabilized with a material such as yttria), alumina, ceria, mullite, zircon, silica, silicon nitride, hafnia, and certain zirconates, borides and nitrides. The columns are said to be about 0.1 mils in cross-section, with thicknesses of 1 to 50 mils. The EBPVD coating process is preferably carried out in such a way as to avoid reduction of the ceramic; a stoichiometric oxide coating (not deficient in oxygen) is desired. One advantage of these columnar coatings is that they are pure refractory without metal alloy inclusions.
The columnar morphology used by other workers, however, has drawbacks. Among them is wear caused by the direct exposure of the refractory to the extremely hot and erosive gas present during gas turbine engine operation. Spalling may be reduced by the ability of-the individual micro-columns of refractory to flex relative to each other without cracking, but the columns remain exposed to erosion due to particulates in the surrounding air or combustion gas.
Diamond, diamond-like carbon and diamond-like hydrocarbon coatings have been employed both to provide hard faces on engineered materials and as erosion-resistant coatings on articles made from such materials. Typically such diamond films and/or particles are applied using some form of chemical vapor deposition (CVD) process. The high thermal conductivity of diamond, however, has resulted in its being regarded in the past as unsuited for applications requiring thermal insulation, as in coating internal parts of gas turbine engines.